The present invention relates to a ball-grid-array-type semiconductor device and its fabrication method and an electronic apparatus, particularly to an art effectively applied to an art for improving the reliability of external terminal connection of a ball-grid-array-type semiconductor device secured to a mounting board.
Because functions of electronic units have been improved, it is requested that a semiconductor device such as an IC (integrated circuit arrangement) or an LSI (large-scale integrated circuit arrangement) is provided with more external terminals (change to multiple pin). A QFP (Quad Flat Package) in which a lead is protruded from four sides of a rectangular sealing body (package) has corresponded to the change to multiple pin by decreasing the intervals between its external terminals to, for example, 0.4 mm and then 0.3 mm in order.
Moreover, as a package structure capable of decreasing the pitch between external terminals, a pin-grid-array-type semiconductor device and a ball-grid-array-type semiconductor device (hereafter also referred to as BGA-type semiconductor device) are developed.
In the case of a BGA-type semiconductor device, an external terminal serves as a ball electrode. The ball electrode is constituted with a metallic ball which does not deform or a solder ball which fuses and deforms.
The ball electrode is soldered to a land of a mounting board. Moreover, the mounting height of a semiconductor device is determined by the diameter of the solder ball (low-fusion-point solder ball which fuses at the time of reflow or high-fusion-point solder ball which does not fuse at the time of reflow) or that of the metallic ball.
A BGA-type semiconductor device is described in "Electron Materials", the April 1997 issue issued on Apr. 1, 1997, pp. 103-105, edited by KOGYOCHOSAKAI (transliterated).
Moreover, the structure of an external terminal comprising a solder ball or the like and the external-terminal structure when mounted are disclosed in published patent journal No. "5-508736" and the official gazettes of Japanese Patent No. 2500109 and Japanese Patent Application Laid-Open No. 8-31974.
A BGA-type semiconductor device has a structure in which bumpy external terminals (external electrodes) are arranged on the back of a wiring board (package substrate) like an array.
As one of BGA semiconductor devices, a structure is known in which solder balls or metallic balls are arranged like an array as external terminals on the back of a package substrate made of low-temperature co-fired ceramic (LTCC)
The present applicant also develops a BGA-type semiconductor device using an LTCC plate. FIG. 12 is a locally-cut-out sectional view of a BGA-type semiconductor device developed by the present applicant. Moreover, FIG. 13 is an enlarged sectional view showing a ball-electrode support portion of the mounting structure of a BGA-type semiconductor device developed by the present applicant.
As shown in FIG. 12, a BGA-type semiconductor device 1 has a rectangular flat package substrate 2 comprising a multilayer wiring structure in which the top (principal plane) center is recessed in two stages. The package substrate 2 is constituted with a low-temperature co-fired ceramic multilayer substrate.
A semiconductor chip 5 is secured to the bottom of a deep recess 3 of the package substrate 2 through a joining material 4. Moreover, wiring 7 is provided for the bottom of a shallow recess 6. Furthermore, the front end portion extending toward the deep recess 3 of the wiring 7 is connected with a not-illustrated electrode of the semiconductor chip 5 by a conductive wire 9.
Furthermore, an insulating sealing resin 10 is injected into the recesses 3 and 6 and cured so as to cover the semiconductor chip 5 and the wire 9.
Furthermore, external terminals (external electrodes) 11 respectively constituted with a ball electrode are arranged like an array on the back (bottom) of the package substrate 2. The external terminal 11 comprises an electrode 15 constituted with wiring formed at the bottom of the package substrate 2 and a metallic ball 17 secured onto the electrode 15 through a pedestal layer 16 made of low-fusion-point solder. The metallic ball 17 is made of, for example, oxygen-free copper.
Furthermore, a plated film 21 is formed on the surface of the electrode 15 in order to improve the adhesiveness of solder (see FIG. 13). In the case of the plated film 21, the lower layer is made of a nickel layer 22 and the upper layer is formed with a gold layer 23.
Furthermore, the wiring 7 and joining material 4 to which the wire 9 of the package substrate 2 is connected are electrically connected to the bottom electrode 15 through a via 19 extending in the longitudinal direction and an internal wiring 20 extending in the horizontal direction in the package substrate 2.
Furthermore, because the pedestal layer 16 is fused once when the metallic ball 17 is set, layer 16 is widely wet along a part of the spherical surface of the metallic ball 17 and the circumferential surface of the layer 16 forms a curved surface between the metallic ball 17 and the electrode 15.
This type of the BGA-type semiconductor device 1 is built in a mounting board of an electronic apparatus.
A printed wiring board (PWB: FR-4) is generally used as a mounting board to be built in an electronic apparatus.
A BGA-type semiconductor device is mounted on paste solder printed by a metal mask by positioning the BGA-type semiconductor device so that external terminals of the BGA-type semiconductor device overlap, for example, lands on the surface of a mounting board, and then reflowing low-fusion-point solder previously provided for the surface of the land (ultimate temperature of 240.degree. C., 180.degree. C. for 40 sec), and securing the external terminals to the lands respectively. The solder on the land surface uses eutectic solder. In this case, because the top and the bottom of the metallic ball 17 are made of low-fusion-point solder (pedestal layer 16 and solder layer 33) as shown in FIG. 13, the ball 17 automatically moves to a stable position due to the surface tension of the solder and securely electrically connects a land 31 of a mounting board 30 with the electrode 15.
Moreover, the surface of the package substrate 2 is provided with a glass layer 12. The electrode is formed with a conductor layer 13 exposed from the glass layer 12. Furthermore, the surface of the mounting board 30 is also provided with an insulating film 32. The land 31 is formed at a portion exposed from the insulating film 32.
According to the method for mounting a BGA-type semiconductor device, the above reflow is performed at a temperature lower than the fusion temperature of high-fusion-point solder and higher than the fusion temperature of low-fusion-point solder. Therefore, the metallic ball 17 made of high-fusion-point solder does not fuse and the BGA-type semiconductor device 1 is kept at a predetermined height by the metallic ball 17. Thus, it is possible to prevent an electrode short circuit from occurring due to a solder bridge between lands.
Moreover, the present inventor finds in a reliability test (temperature cycle test) of a mounted BGA-type semiconductor device that the joint between the land 31 and the electrode 15 may deform so as to inversely tilt at the both ends of the package substrate 2 or the portion of the package substrate 2 for supporting the external terminal 11 may be cracked. Moreover, it is found that extreme one of these phenomena causes electrode disconnection.
FIG. 14 is a schematic view of a package substrate showing a state of the joint between a BGA-type semiconductor device developed by the present applicant and a mounting board after mounting the semiconductor device on the mounting board and performing a heat cycle test, which is obtained by tracing a microphotograph.
The package substrate of the BGA-type semiconductor device used for the above heat cycle test is a low temperature co-fired ceramic multilayer substrate which is a square substrate having a thickness of 0.9 mm and a side length of 15 mm. An external terminal has an array structure obtained by arranging metallic balls (diameter of 0.5 mm) at a pitch of 1.27 mm in 11 rows and 11 columns. The thickness of the package substrate including external electrodes is 1.6 mm. The printed wiring board is constituted with a four-layer structure plate (FR-4) having a thickness of 1.6 mm.
Moreover, the temperature cycle test is performed by repeatedly changing the temperatures of -40.degree. C. and 125.degree. C. every 30 min and the state in FIG. 14 is a state after 200 cycles.
A joint 35 between the land 31 of the mounting board 30 and the electrode 15 of the package substrate 2 of the BGA-type semiconductor device 1 further tilts at the both ends of the package substrate 2. The joint 35, though not illustrated in FIG. 14, comprises the metallic ball 17, and the pedestal layer 16 and solder layer 33 which serve as the top and bottom low-fusion-point solder layers.
The torsion and tilt of the joint 35 occur due to the difference of thermal expansion coefficient between the package substrate 2 constituted with a low-temperature co-fired ceramic multilayer substrate and the mounting board 30 constituted with a printed wiring board. That is, the thermal expansion coefficient .alpha. of the low-temperature co-fired ceramic multilayer substrate is approx. 5.5 ppm/.degree. C. but the thermal expansion coefficient .alpha. of the printed wiring board (PWB: FR-4) is approx. 13 ppm/.degree. C. which is greatly different from the thermal expansion coefficient of the multilayer substrate.
In the heat cycle test, the solder (solder layer 33) on the land 31 of the mounting board 30 is dragged toward the center of the package substrate 2 due to shrinkage of the mounting board 30 having a shrinkage ratio larger than that of the package substrate 2 at a low temperature and moreover dragged toward the both ends of the package substrate 2 at a high temperature. Though the solder layer 33 on the land 31 supporting the metallic ball 17 is easily dragged toward the center of the package substrate 2, it does not easily return to the both ends of the package substrate 2 because the tensile strength of solder decreases and simultaneously, an elongation occurs at a high temperature. Therefore, the phenomenon shown in FIG. 14 occurs. Eutectic solder shows an approx. tenfold strength difference at a high temperature and a low temperature. Particularly, though the eutectic solder shrinks in a negative temperature region but the deformation of the solder is not easily restored in a high-temperature region because the tensile strength of the solder decreases.
When a stress is repeatedly applied to the joint 35, a crack 34 occurs in the brittle package substrate 2 (low-temperature co-fired ceramic multilayer substrate). The crack 34, as shown by the two-dot chain line in FIG. 13, occurs along the direction of a curved surface like a circular arc so as to shave the electrode 15 connecting with the joint 35 to deteriorate the reliability of the electrode or disconnect the electrode.